Michael Flynn is only the latest shoe to drop. We know others are coming.

Late Tuesday, special counsel Robert S. Mueller III filed his much-anticipated sentencing memorandum in the case of former national security adviser Michael Flynn. Large portions of Mueller’s memo were redacted because they relate to ongoing investigations. But reading between the blacked out lines, the latest salvo from the special counsel suggests the White House should be very uneasy about what may be still to come.

Flynn was the nearly forgotten cooperator. He pleaded guilty fairly early in Mueller’s investigation, and was the first major Trump administration official to agree to cooperate with the prosecution. But, after his guilty plea, we heard little about him. The public’s attention turned to events such as the trial and guilty plea of former Trump campaign chairman Paul Manafort , as well as the guilty pleas made by Michael Cohen, President Trump’s former personal lawyer.

But the documents filed on Tuesday reveal that Flynn has been busy behind the scenes. Prosecutors told the court that he participated in 19 interviews with the special counsel’s office, with lawyers from other Justice Department offices, and provided documents and communications. He likely also testified before one or more grand juries, though that description of his cooperation was redacted.

Mueller informed the court that Flynn provided “substantial assistance” to those criminal investigations, a term of art prosecutors use when they agree a defendant should receive sentencing credit for his cooperation. Flynn faced only a maximum of six months in prison even before his cooperation; prosecutors said that, based on his assistance, a sentence that included no jail time would be appropriate.

Flynn had pleaded guilty to lying to the FBI about his conversations with the then-Russian ambassador during the presidential transition. Those conversations are relevant to the central issue in Mueller’s inquiry: a potential conspiracy involving Russian officials and members of the Trump campaign. For example, prosecutors are likely exploring whether there was a quid pro quo arrangement where Trump officials agreed to ease sanctions against Russia, or to take other favorable actions in exchange for Moscow’s help during the presidential election or its help related to business dealings in Russia. The memo seems to confirm that Flynn has now told prosecutors whatever he knows about interactions between the Trump team and Russia.

But the redacted portions of the sentencing documents suggest there are at least two other investigations in which Flynn has cooperated. One has a separate (redacted) heading in the memo; that, plus the reference to assisting prosecutors from other Justice Department offices, suggests the former national security adviser is cooperating in a criminal investigation being handled outside Mueller’s shop. This could be another Trump-related investigation spun off from Mueller’s inquiry, such as the one going on in the U.S. Attorney’s Office for the Southern District of New York that led to the first guilty plea from Cohen. Or it could be something else entirely that we don’t yet know about.

A second heavily-redacted portion of the memo appears to refer to Flynn’s cooperation in an area of the special counsel’s investigation other than interactions between Trump officials and Russians. Again, we can’t be certain, but it seems likely this refers to Mueller’s investigation of possible obstruction of justice by the president. Flynn was at the center of much of that alleged obstruction; for example, former FBI director James B. Comey reported that Trump asked him to back off on the investigation of Flynn after it became apparent that he had lied to the FBI about his actions on behalf of the president. It make sense that Flynn had information relevant to the obstruction investigation to share with the special counsel’s office.

But the really striking thing driven home by this filing is how much we still don’t know about what Mueller has discovered. Flynn’s cooperation was extensive, and information about the majority of it is still under seal. We recently learned that Cohen has met with prosecutors for about 70 hours, while former White House counsel Donald McGahn reportedly spoke to prosecutors for 30 hours. Former Trump deputy campaign manager Rick Gates recently had his sentencing postponed because he, too, continues to cooperate in several ongoing investigations.

That’s a whole lot of information Mueller has gathered from members of the president’s inner circle. We have yet to see the fruits of almost any of this cooperation. Those who thought Mueller might be close to wrapping up his investigation may need to think again. Recent developments demonstrate that prosecutors, both in the special counsel’s office and elsewhere, are still actively pursuing a number of investigations that have yet to be fully revealed. There are likely a number of potentially very large shoes still to drop.